Is there a way to connect to a bluetooth headset as it was a headphone?
I mean, I use a BH-504 and while it is great for listening to music I am not satisfied with it's mic
and I would prefer to use it only as a connected headphone and in case of a call use the mic of the N900.

I only found a way to disable the HFP profile, so that now I don't accidentally call the last dialled number. The rest of the buttons work ! (next, previous, pause). Some may want this , some may not. The setting is in /etc/bluetooth/audio.conf but it is not device specific.
I guess I need now to also disable HSP profile.

More information as you go, so others can change their settings would be nice.

so far, a little guide for reducing bluetooth functionality!
If you want to only disable the dial last number functionality on connected headsets:
become root and edit file /etc/bluetooth/audio.conf
there is a line there
HFP=true
change that to
HFP=false
and reboot.

if for some reason you want to disable HFP/HSP profiles so that you may use bluetooth headphones for music listening, but when a call comes you will have to pick up your N900 and use it like it was not connected
change line
MaxConnected=1
to
MaxConnected=0
and reboot.

My original question stays unanswered.
if you want to revert changes you have to also reconnect your device I believe.

Useful, as the mic on my BT headphones sucks big time.